Conventional radio signals are produced by amplitude- or frequency-modulation of a sinusoidal carrier with a baseband signal. Signals from different sources can be distinguished by using different carrier frequencies. Selective reception is accomplished by means of filters that will resonate with the carrier having a chosen frequency, but not with any other carrier. This is the basis of selective transmission and reception of conventional radio signals.
For certain applications, such as a radar that can penetrate seawater or absorbing materials, one cannot use sinusoidal carriers. Instead, one can use signals that consist of a sequence of pulses--which for simplicity may be visualized as rectangular binary pulses--and assign different sequences of pulses to different users. Selective reception then requires that a particular sequence of pulses is recognized in the presence of noise and many other signals, while all other pulse sequences are rejected. It is known that this can be accomplished in principle by a sliding correlator. Such correlators have been built for years for pulse durations of 100 ns and longer. However, for carrier-free radar, one needs sliding correlators that can process pulses with a duration of about 1 ns or less.
Sliding correlators for the processing of analog signals--which may be originally sequences of digital pulses that have been distorted and have superimposed noise as well as unwanted signals--generally use either analog delay devices or acousto-optical devices. In either case, the minimum duration of pulses that can be processed is on the order of 100 ns or more, while the theoretically foreseeable minimum duration is a few nanoseconds. It is important to observe that we are concerned with analog signals; sliding correlators for digital pulse sequences or "characters" (as used in computers) can work must faster. The reason for this difference in speed is that the digital pulse sequences require digital delay circuits, usually referred to as digital shift registers, rather than analog delay devices which contain typically relatively slow electro-acoustic, acousto-optic, or charge-coupled devices.
Accordingly, it is an object of the present invention to provide a sliding correlator which can process pulses of 1 ns duration, or less.
Yet another object of invention is to use digital delay devices instead of analog delay devices in sliding correlators for analog signals.
The foregoing and other objects and advantages will be more fully understood from the detailed description below, which should be read in conjunction with the accompanying drawing.
The foregoing and other objects of the invention are achieved with a digital sliding correlator having means for producing a series of time-shifted stored copies of a digital character, means responsive to said shifted, stored copies of the character for multiplying said shifted, stored copies by the input signal or the amplitude-inverted counterpart of the input signal, a first set of integrators and a second set of integrators, means for supplying to the first set of integrators first portions of said multiplied signals and for supplying to said second set of integrators second portions of said multiplied signals, and means for combining the outputs of said integrators.